Gas turbine engines generally include a coumbustion chamber for generating a gas flow. Turbine blades connected to a rotor are located aft of the combustor and within the gas flowpath so as to extract useful energy from the gas flow. In order to optimize the amount of energy extracted, an array of vanes is typically interposed between combustor and turbine blades to turn the gas stream. By imparting a circumferential component to the flow, higher turbine blade speeds are attainable.
In many gas turbine engines, it is desirable to rotate the vanes to control the gas flow between the vanes. For example, in aircraft engine applications, power requirements may differ depending on the flight condition. Consequently, an array of fully variable turbine vanes may be advantageously employed. However, to be fully effective, any variable turbine array must be mounted so as to minimize leakage of the gas from the designated flowpath and around the airfoil.
In a typical engine containing an annular combustor, an annular casing with inner and outer casing walls surrounds the combustor. The casing defines a flowpath between the combustor and casing for air to cool the walls of the combustor. The casing extends beyond the aft end of the combustor and provides the structure to which the vane array may be attached. For example, U.S. Pat. No. 3,663,118--Johnson shows such a mounting arrangement.
The problem with using the outer and inner casing walls for support is that they exhibit differential rates of thermal growth due to the inner casing wall being from 200.degree. to 300.degree. hotter than the outer casing wall. Thus, the inner wall exhibits greater axial movement due to thermal expansion than the outer wall. This may stress seals thereby creating gas flow leakage paths. Futhermore, the turbine vane might bind on the surrounding shroud as their axes become misaligned.
An alternative to mounting the vane array on inner and outer casing walls is to mount it solely on the outer wall in a cantilever arrangement. A floating seal would be provided near the vane root to prevent leakage. The problem with this solution is that it requires an unduly large structure to counteract the bending stress on the vanes induced by the gas flow. Moreover, due to the differential temperature discussed above, a larger than normal radial gap must be provided to assure non-binding at all conditions. These gaps are controlled by floating seals which have the potential to leak relatively large quantities of gas.